1. Field of Industrial Application
This invention relates to a sodium percarbonate particle having excellent stability and also to a process for preparing the same.
2. Description of Related Art
Sodium percarbonate is well-known as a bleaching or oxidizing agent. It is commonly manufactured by the reaction of sodium carbonate with hydrogen peroxide. As its general formula 2Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2 implies, it comprises an addition compound of sodium carbonate with hydrogen peroxide. Although sodium percarbonate is somewhat inferior in bleaching power at ordinary temperature to chlorinated bleaches, it can be used in bleaching textile pieces with colored patterns, does not damage the fabric, and causes no yellowing. These and other advantages have allowed it to enjoy widespread use, alone or in admixture with detergents, as oxygenated bleaching agents for household or business use.
The reason why an interest is attached to the use of sodium percarbonate in applications such as general detergents and domestic bleaches is that its decomposition products are not pollutants and can be used in whatever way desired without any problem.
However, sodium percarbonate has a serious drawback in that it is rather poor in storage stability as compared with sodium perborate and fairly rapidly loses its available oxygen during storage. Since sodium percarbonate has a very high affinity towards water, it becomes wetted on its surface even with a small amount of moisture and begins decomposing. Iron, copper, manganese, cobalt, and other such ions, if present in the system, accelerate its decomposition, making it less stable than sodium perborate. When sodium percarbonate, alone, is stored in a closed vessel, it proves to be as stable during storage as sodium perborate. When blended with a detergent or left in an open vessel, however, it exhibits inadequate storage stability because of its high hygroscopicity, despite its good solubility.
It is therefore desired to provide a sodium percarbonate which does not decompose rapidly upon standing in an open vessel and, when blended with whatever detergent or bleach composition, does not present any storage stability problems,
Many different researches have been conducted with regard to the stabilization of sodium percarbonate and various approaches have been proposed. Particularly, a large number of proposals have been made with respect to coating the surface of a sodium percarbonate particle with an inorganic matter.
For instance, Japanese Patent Publication No. 31839/1972 discloses sodium percarbonate surface-coated with sodium perpyrophosphate; U.S. Pat. No. 3,977,988, sodium percarbonate surface-coated with a silicate-silicofluoride mixture; Japanese Patent Publication No. 5875/1978, sodium percarbonate surface-coated with a silicate-glycine mixture; U.S. Pat. No. 4,105,827, sodium percarbonate surface-coated with a crystallized mixed salt consisting of sodium carbonate and either sodium bicarbonate or sodium sulfate; U.S. Pat. No. 4,194,025, sodium percarbonate surface-coated with dehydrated sodium perborate and sodium silicate; U.S. Pat. No. 4,325,933, sodium percarbonate surface-coated with an alkaline earth metal salt; U.S. Pat. No. 4,156,039, sodium percarbonate surface-coated with dehydrated sodium perborate; Japanese Patent Laid-Open No. 193999/1984, sodium percarbonate surface-coated with a borate-silicate mixture; Japanese Patent Laid-Open No. 194000/1984, sodium percarbonate surface-coated with a borate-magnesium compound mixture: Japanese Patent Laid-Open No. 118606/1985, sodium percarbonate surface-coated with a mixture of boric acid or borate and a water repellent agent; and U.S. Pat. No. 4,526,698, sodium percarbonate surface-coated with a borate. These sodium percarbonates coated with inorganic salts are fairly improved in stability but not to a satisfactory extent.